how to manipulate private variable between classes

<p>I'm trying to build a stock inventory and cash register simulation, in the code I have 2 class, first is Inventory and second is Cash. Both have their own properties and method, when I'm trying to sell item the _balance property is giving Error undefined _balance and the _balance is still 0. How should I fix it? Just in case this question have been already asked please just give the link don't mark as duplicate. I just can't find it.</p><pre><br /><code><?php<br /> class Inventory {<br /> public $_name;<br /> public $_price;<br /> public $_stock;<br /> public function _addStock($item,$itemIn)<br /> {<br /> $this->_stock += $itemIn;<br /> }<br /> };<br /><br /> $item2 = new Inventory;<br /> $item2->_name = "item2";<br /> $item2->_price = 5000;<br /> $item2->_addStock('item2',100);<br /> $item1 = new Inventory;<br /> $item1->_name = "item1";<br /> $item1->_price = 1000;<br /> $item1->_addStock('item1',100);<br /><br /> class Cash {<br /> private $_balance = 0;<br /> public function _display(Inventory $_name)<br /> {<br /> echo $_name->_price;<br /> }<br /> public function getBalance()<br /> {<br /> echo $this->_balance;<br /> }<br /> public function sell(Inventory $_name, $amount)<br /> { <br /> return $_name->_stock -= $amount;<br /><br /> return $_balance += $_name->_price*$amount;// Error $_balance undifined<br /> } <br /> };<br /> $cash = new Cash;<br /> print_r($item2);<br /> echo "<br/>";<br /> print_r($item1);<br /> echo " <br/>Balance :<br/>";<br /> $cash->getBalance();<br /> echo "<br/>item1 price :<br/>";<br /> $cash->_display($item1);<br /> echo "<br/>item2 price :<br/>";<br /> $cash->_display($item2);<br /> echo "<br/>item1 sold 12 pcs<br/>";<br /> $cash->sell($item1,12);<br /><br /> $cash->sell($item2,12);<br /> echo "<br/>item2 sold 12 pcs <br/>";<br /><br /> echo "<br/> current stock :<br/>";<br /> print_r($item2);<br /> echo "<br/>";<br /> print_r($item1);<br /><br /> echo "<br/> current balance :<br/>";<br /> $cash->getBalance();<br /> ?><br /></code><br /></pre><br /><h2>Click here view more answers <a href="%20http://stackoverflow.com/questions/34525126/how-to-manipulate-private-variable-between-classes">http://ift.tt/1mhPAeU</a></h2>